There’s something far more important about S.T.A.L.K.E.R., however, and that’s the kind of game it was trying to be – a singularity that is both alien to and coexistent with the tradition of Western shooters. It took everything it could from Quake, Half-Life, and Unreal, but the alloy it forged with them was shot through with other elements: Eastern pessimism, RPGish sentiment. Crucially, it is open and non-linear, like an RPG. It is also quest-based, rather than objective-orientated, which means you are not necessarily rushing to the next waypoint, or following the nearby NPC, but instead deciding if you should wander into a side-quest, or make an exploratory digression. It is a shooter paced and spaced like an RPG, leading to an experience that feels wide-open by comparison with other shooters, while at the same time having brutal and violent FPS combat mechanics. There’s never any doubt that this is a game about guns, but the consequence of how it deals with everything else makes the experience unique.

It is also terrifying. Few games have managed to convey such a sense of threat in enemies or environments. The shooters that overtly reach for horror – I’m looking at you, FEAR games – come off looking trite next to Stalker’s claustrophobic, howling terrors.

GSC’s key success in making all this work, I believe, was in the execution of their “A-Life” concept, which was to have a great deal of randomised activity taking place in the world, whether or not you chose to interact with it. Packs of blind dog hunted, Stalkers wandered lonely paths, bandits lay in wait for travellers – travellers who may or may not come along that road. That’s not to say it had great AI, but rather that the illusion of a world it created was so potent.